As Direct Access Storage Devices (DASD) such as disk drives have been reduced in size, the actuators contained within the DASD have been similarly refined to result in higher density recording onto the disks. As the recording density has increased, the actuator positioning capability for read/write head position resolution either has reached or is rapidly approaching a track density limit. The geometry and form factor of the disk drive limit the relative lever arm length, and further density increases are thereby limited unless alternative approaches are implemented.
The typical actuator is formed of a drive armature arm, an actuator arm, and a pivot therebetween. The actuator must be confined within the DASD and be movable with respect to the disk similarly located within the DASD. Any increase in the armature arm length which might serve to increase the resolution of the positioning of the actuator arm is severely limited by the overall size limitations placed on the DASD. Similarly, the travel of the armature arm of the actuator is limited by the structure of the DASD. Presently the DASD disk surfaces and recordability are so improved as to permit a higher recording density. Also, optical recording disks permit very high recording densities, allowing the extremely close placement of the recording tracts relative to each other on the disk of the DASD. With the disks having very fine resolution recording capability, the actuator and the actuator positioning resolution, more particularly, becomes the recording limiting factor.
The actuator positioning resolution is further limited by forces heretofore not considered significant with respect to larger disk drives. Stiction forces exist in the bearing of the actuator and thus limit the reduction of the driving forces which are exerted on the actuator to cause the actuator to move. Once stiction forces are overcome, the bearing permits very low forces to drive the actuator through further movement. With the recording density of typical disk drives in the order of 4,000 to 6,000 tracks per inch (150 tracks per mm), the movement required to move to an adjacent recording track is 0.00025 inches (0.0067 mm).
In order to overcome the stiction forces, a significant magnitude force must be exerted on the armature arm of the actuator. The force exerted must exceed any resistive force in the bearing caused by stiction. Essentially no other forces resisting movement of the actuator exist. Once the stiction force has been overcome and the armature is then moving, the forces for overcoming the dynamic friction of movement are very significantly less than that required to overcome the stiction forces. It is therefore necessary to first overcome the stiction forces and then immediately slow and stop the armature movement so that the distance traversed is minimal and permits the desired recording track density on the disk. Since the force exerted in overcoming the stiction resistance in the bearing or bearing cartridge of the armature is significantly higher than that required to overcome the dynamic friction of the bearing and since the exact moment of overcoming the stiction force cannot be predicted and anticipated, the effect is that the armature will translate rotationally about its pivot axis some minimum distance before the armature arm can be acted upon and stopped. The effect of this is what is known as and referred to as a "jump." One may readily understand that the resolution of the recording positions of the actuator are limited by the magnitude of the jump for a particular design.
In the ongoing effort to reduce the size and particularly the thickness of the DASD, the size of the actuator arm and the thickness of the material from which it is made have been reduced to the point that warpage and deflection become significant design considerations. In addition, some approaches to deflecting the tip of the actuator arm relative to the main portion of the actuator arm structure have included forming serpentine portions which are easily deformed, bending the load beam or distal portion thereof and/or hinging the actuator arm and load beam. These approaches appear to increase the possibility of warpage and undesirable deflection of the load beam and particularly the positioning of the slider as a result of the warpage or deflection. Other approaches to displacing, bending or deforming the load beam of the actuator arm have included the use of piezoelectrically driven displacements of the actuator arm.
The inherent weakening of the load beam or the designing the load beam to possess sufficient flexibility to permit deflection under the influence of piezo-electric elements introduces additional warpage and instability which, in turn, raises the possibility of damage to the disk of the disk drive through undesired and uncontrolled contact between the slider and the disk surface. The weak or weakened portion of the arm is deformed by a piezo-electric transducer (PZT) element which is attached at one point to a more rigid portion of the arm and at a second point to the weakened portion or movable portion. In some of the implementations the PZT element is a bi-morph element which induces bending both of the element and of the segment of the actuator arm to which it is attached, thereby causing the deformation in the actuator arm and the commensurate movement of the slider.
The structures of the prior art are generally used for and are particularly adapted both to tracking of the data track to insure that the head of the slider remains over and centered on the data track of the disk and as well to performing small seeking operations while relying upon the actuator for the positioning of the load beam in the proper position for accessing a particular data track. The control of the piezoelectric transducers is dynamic in nature to perform effective track following thereby making the small adjustments necessary to continuous position the recording head over the data track.